Electricity supply in Australia is largely based on remote, centralised fossil-fuelled power stations. Several factors are emerging that will change this platform to one of a more distributed, and potentially intermittent, generation. These include a desire from governments and consumers to reduce carbon emissions, increasing costs of conventional fossil-based energy, and a need to improve network quality and reliability in some fringe and constrained regions. This growing move to distributed and intermittent systems requires a concurrent development of energy storage technology if reliability and quality of supply are to be maintained. Indeed, grid connected energy storage is now acknowledged to be a key component of future electricity supply infrastructure. Various technologies are being considered for grid and transport storage applications, including lithium-ion batteries, sodium-sulfur batteries (NGK Japan), flow batteries, compressed air systems, flywheels, supercapacitors and many more. Flow batteries have long been considered to be the most suitable storage technology for utility applications due to their potential long life, deep discharge characteristics and potentially low manufacturing cost. Flow batteries differ from other battery technologies in that the electrolyte is pumped over the electrodes, which remain electrochemically inert, storing charge through a change in oxidation state (e.g. vanadium redox) or through an electrodeposition such as the zinc-bromine battery. Of these, the zinc-bromine battery offers a solution to most of the problems that have challenged flow battery systems and is considered a highly prospective technology.
A zinc-bromine battery consists of two cells separated by a permeable membrane through which a zinc bromide/bromine electrolyte is circulated (see, e.g., FIG. 1). During the charging step, zinc is electroplated onto the carbon anode, and Br2 is evolved at the carbon cathode. A complexing agent in the electrolyte, N-ethyl-N-methylpyrrolidiniumbromide (MEPBr), is used to reduce the reactivity and vapour pressure of the elemental Br2 by complexing the majority of the Br2 to MEPBr, forming a so-called polybromide complex (MEPBrn). This minimises the self-discharge of the battery and significantly improves the safety of the system. This complex is removed from the stacks via the flowing electrolyte and is stored in an external reservoir. On discharge, the complex is returned to the battery stacks by the operation of a valve or a third pump. Zinc is oxidized to zinc ions on the anodes; the Br2 is released from the complex and subsequently reduced to Br− ions on the cathodes.
While operational and economic for some applications, existing zinc-bromine battery technology currently only operates at 15% of the theoretically achievable (based on ZnBr2 solubility) specific energy due to sub-optimal electrode design, poor fluid dynamics and the inefficient two-phase fluid, gravity-separated complexing of Br2. This limits the battery to non-transport and low specific energy and energy density applications. Many of the disadvantages with current zinc-bromine battery technology relate to problems with efficiently storing and/or transporting Zn2+ and Br2/Br− in the electrolyte solution. For example, current battery systems are limited in their specific energy output by the complexing capacity of bromine sequestering agents (BSAs) in the electrolyte, and an ion-selective membrane is needed in current battery systems to prevent a direct reaction between the zinc electrode and bromine that would otherwise lead to the battery shorting out.